The present invention relates to a telephone system and more specifically to a transmission level regulating method for a telephone system connectable to both analog and digital communication networks.
As an example of telephone system of this type, there exist key telephone system connectable to both the integrated service digital network (ISDN) and the (analog) public subscriber telephone network (PSTN). FIG. 1 shows a typical system configuration of this telephone system. On a key service unit 1, there are mounted an ISDN interface unit 5 connected to an ISDN line 3, a PSTN interface unit 9 connected to a PSTN line 7, an extension terminal interface unit 15 connected to a line 13 from an extension terminal 11, etc. These interface units are connected to a control unit 20 for controlling the entire key service unit and channel switching, via a data highway 17 serving as a time-sharing transmission line for control data and a speech highway 19 serving as a time-sharing transmission line for speech PCM (pulse code modulation) codes.
Here, there exists a loss of 4 dB on an average between a subscriber for the PSTN line 7 and a terminal office. In addition, some loss is inevitably produced in a 2-4 line switching circuit and a CODEC (coder/decoder) of an office line (PSTN) interface unit 9. Further, in general the key service unit 1 is so designed that there exists a loss of about -1 dB between the office line (PSTN) interface unit 9 and the individual telephone set interface unit 15.
On the other hand, no loss exists between a subscriber for the ISDN line 3 and a terminal office. Further, the conventional ISDN interface unit 5 performs only signal representation transformation between ternary codes and binary codes, speed conversion between 64 Kb and 2,048 Mb, multiplexing, separation, etc. in general, without performing conversion with respect to the speech PCM codes.
As a result, when an extension telephone set 1 is connected to an outside line telephone set such as an analog telephone set 23, there arises the following problem: Since both the ISDN and PSTN lines are connected to the key service unit 1, although telephone conversation can be made through any one of the two lines, there exists a difference in speech (transmission) level between the two lines, because the loss through the PSTN line 7 is different from that through the ISDN line 3, as already explained. In other words, the speech level differs according to the used network, when conversation is made with the same person. The similar difference in the speech level occurs between when the extension telephone set 11 is connected to a digital telephone set 21 and when the extension telephone set 11 is connected to an analog telephone set 23. In addition, the difference in speech level changes being subjected to the influence of installation conditions of the lines 3 and 7 and the key service unit 1.
As described above, in a telephone system connected to both the analog and digital communication networks, there exists a problem in that the speech (transmission) level differs according to the connected network and further the level difference varies according to the device installation conditions, thus resulting in a difficulty in maintaining an appropriate speech level all the time.